Wednesday, July 30, 2003
Copyright © Las Vegas Review-Journal

Scientists plotting 'shake map'

Valley experiment to help show where earthquakes might surface

Don Rock, an electronics engineer from California's Lawrence Livermore National Laboratory, prepares to install a seismic wave sensor Tuesday at Hyde Park Middle School. Photo by Clint Karlsen. Click image for enlargement. Graphic by Mike Johnson.

Don Rock lowered a heavy, metal cylinder into a freshly dug hole Tuesday at Hyde Park Middle School where scientists hope it will detect the slightest ground-motion waves traveling through a thick layer of sediments over the Valley View Fault.

Rock, an electronics engineer from California's Lawrence Livermore National Laboratory, then connected the wires from a solar panel and a nearby computer. The system is designed to convert the detector's electronic signals into numerical data.

The so-called seismic recorder, or seismograph, and five others like it located at two other schools and Nellis Air Force Base will be ready next month for one of the more ambitious geology experiments ever conducted in the Las Vegas Valley.

Chemical explosives ranging in size from 1,500 pounds to 50 pounds are scheduled to be detonated around the valley the night of Aug. 11 to simulate shock waves from an earthquake of less than magnitude 2 traveling through the basin.

On the earthquake magnitude scale, a 2 is generally below the threshold that people can feel and not powerful enough, in the case of deeply buried explosives, to cause damage at the surface.

"We're going to try and minimize the noise as much as possible but they might hear a bang about as loud as a firecracker," said Cathy Snelson, assistant professor of crustal geophysics for the Nevada Seismological Laboratory's branch at UNLV.

"The big shots will be loud enough in terms of seismic energy that they will propagate across the basin's entire profile," she said.

"Hopefully, what this project will tell us is what this basin looks like in the subsurface ... and how shaking affects the entire city," Snelson said.

Next week contractors will begin drilling some of the 120-foot-deep holes where the ammonium nitrate charges will be placed.

In addition to the six seismographs, which can detect ground-motion waves from earthquakes locally, regionally or worldwide, another 825 portable recorders will be buried a foot below the surface and 100 meters apart in two lines to, in essence, listen for the explosions' shock waves and generate three-dimensional images of them.

The $150,000 project funded by the Lawrence Livermore lab, the U.S. Geological Survey and the University of Nevada, Las Vegas will help scientists develop what's called a "shake map."

Nevada ranks third among states, behind Alaska and California, for frequency of strong earthquakes. Major metropolitan areas such as Los Angeles, the San Francisco Bay area and Seattle already have shake maps. Not so for the 1.5 million people who live in the Las Vegas Valley.

Earthquakes are caused by the gradual movement of huge, tectonic plates, or rock layers that make up the Earth's crust. This movement builds up strain. An earthquake occurs when the strain is relieved and an enormous amount of energy is released. This happens in much the same way a pencil snaps when bent from both ends at the same time.

Earthquakes are measured by magnitude. With every increase in magnitude, the size of the seismic waves that travel through the crust increase by a factor of 10. In terms of energy released, that translates to a 30-fold increase, which means a magnitude 6.7 earthquake sends out 900 times more energy than a 4.7 earthquake.

Geologists, so far, have charted eight earthquake faults in the Las Vegas Valley, but they aren't sure when they last moved. Some evidence suggests strong earthquakes happen on them about every 10,000 years to 50,000 years in some cases. The last quake on the Decatur Fault, for example, may have occurred anywhere from 1,000 to 15,000 years ago.

Las Vegas geologist Burt Slemmons, professor emeritus at the University of Nevada, Reno, has reported that the fault system at Frenchman and Sunrise mountains has produced about 10 strong earthquakes during the past 125,000 years. It is capable of generating a magnitude-6 to magnitude-7 earthquake based on its 20-mile length and the fact that the fault has shifted before.

That means the Frenchman Mountain Fault could deliver an earthquake on par with the deadly 1994 Northridge quake in California, a 6.7 magnitude.

The Las Vegas Valley also is vulnerable to seismic waves from strong earthquakes that occur as far away as Death Valley and other parts of Southern California.

A report by Snelson earlier this year found that a strong earthquake in Southern Nevada would cause more damage than previously thought, possibly resulting in a substantial increase in previous estimates of 400 deaths, 10,000 injuries and $11 billion in economic losses. That's because the valley is filled with deep layers of sediments more than three miles thick on the northeast side, which would amplify ground shaking as if someone shook a gigantic bowl of Jell-O.

"We're the third most active seismic state, and here in Southern Nevada we've got to start thinking about it," she said.